The organic EL device is a self-luminous device and has been actively studied for their brighter, superior visibility and the ability to display clearer images in comparison with liquid crystal devices.
In 1987, C. W. Tang and colleagues at Eastman Kodak developed a laminated structure device using materials assigned with different roles, realizing practical applications of an organic EL device with organic materials. These researchers laminated an electron-transporting phosphor and a hole-transporting organic substance, and injected both charges into a phosphor layer to cause emission in order to obtain a high luminance of 1,000 cd/m2 or more at a voltage of 10 V or less (refer to Patent Documents 1 and 2, for example).
To date, various improvements have been made for practical applications of the organic EL device. Various roles of the laminated structure are further subdivided to provide an electroluminescence device that includes an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a cathode successively formed on a substrate, and high efficiency and durability have been achieved by the electroluminescence device (refer to Non-Patent Document 1, for example).
Further, there have been attempts to use triplet excitons for further improvements of luminous efficiency, and the use of a phosphorescence-emitting compound has been examined (refer to Non-Patent Document 2, for example).
Devices that use light emission caused by thermally activated delayed fluorescence (TADF) have also been developed. In 2011, Adachi et al. at Kyushu University, National University Corporation realized 5.3% external quantum efficiency with a device using a thermally activated delayed fluorescent material (refer to Non-Patent Document 3, for example).
The light emitting layer can be also fabricated by doping a charge-transporting compound generally called a host material, with a fluorescent compound, a phosphorescence-emitting compound, or a delayed fluorescent-emitting material. As described in the Non-Patent Document, the selection of organic materials in an organic EL device greatly influences various device characteristics such as efficiency and durability (refer to Non-Patent Document 2, for example).
Because phosphorescence-emitting compounds and delayed fluorescent-emitting materials undergo concentration quenching, a charge-transporting compound, or a host compound as it is generally called, is used to support the phosphorescence-emitting compounds or the delayed fluorescent-emitting materials by being doped with the phosphorescence-emitting compounds or the delayed fluorescent-emitting materials. The phosphorescence-emitting compounds or the delayed fluorescent-emitting materials so supported are called guest compounds.
4,4′-Di(N-carbazolyl)biphenyl (CBP) represented by the following formula is commonly used as the host compound (refer to NPL 4, for example).
(CBP)
However, because of the low glass transition point (Tg) of 62° C. and high crystallinity, it has been indicated that CBP lacks stability in the thin-film state. The device characteristics are thus unsatisfactory in situations where heat resistance is needed such as in emitting light of high luminance.
Advances in phosphorescent device studies have promoted further understanding of the energy transfer process between the phosphorescent material and the host compound. Studies found that the host compound needs to have a higher excitation triplet level than the phosphorescent material in order to increase luminous efficiency.
The external quantum efficiency of a phosphorescent device remains at about 6% when the blue phosphorescent material FIrpic of the formula below is doped to CBP to provide the host compound of the light emitting layer. This is considered to be due to the lower excitation triplet level of CBP, 2.57 eV, than the excitation triplet level, 2.67 eV, of FIrpic, making it difficult for the FIrpic to sufficiently confine triplet excitons. This has been demonstrated by the temperature dependence of the photoluminescence intensity of a thin film produced by the doping of CBP with FIrpic (refer to NPL 5, for example).
It has been known that in the case where the delayed fluorescent-emitting materials are used, the excitation triplet level of the host compound needs to be higher than the excitation triplet level of the light-emitting material (refer to NPL 6, for example).

The host compound 1,3-bis(carbazol-9-yl)benzene (mCP) of the formula below is known to have a higher excitation triplet level than CBP. However, as with the case of mCP, mCP has a low glass transition point (Tg) of 55° C. and high crystallinity, and lacks stability in the thin-film state. The device characteristics are thus unsatisfactory in situations where heat resistance is needed such as in emitting light of high luminance (refer to NPL 5).

It has been found from the studies of host compounds of higher excitation triplet levels that doping an iridium complex to an electron transporting host compound or a bipolar transporting host compound can produce high luminous efficiency (refer to NPL 7, for example).
In an organic EL device, light emission is obtained in a light emitting layer through recombination of charges injected from both the electrodes, and for providing an organic EL device with high efficiency, a low voltage operation capability, and a long lifetime, by using a bipolar transporting host compound, a device excellent in carrier balance can be obtained, in which electrons and holes can be efficiently injected or transported to the light emitting layer, and can be efficiently recombined.
As described above, in order to improve the luminous efficiency of a phosphorescent device or a delayed fluorescent-emitting device in actual settings, a light-emitting-layer host compound is needed that has a high excitation triplet level, and high thin-film stability.